The present invention relates in general to an optical tag, and more particularly, the present invention provides a small, thin, environmentally robust, long-lived, modulated optical retro-reflecting tag and a long range interrogation system.
Tagging technology provides a means to covertly but unambiguously mark a person, target or location so that it may be identified from a distance with a specialized sensor. The use of a tag greatly eases or eliminates target detection, identification, and discriminating problems. For example, in a precision strike application, a tag is applied to a vehicle or location in advance of the strike and a sensor (possibly organic to the strike weapon) uses the tag to simplify the process of finding and identifying the target. This allows for an autonomous terminal engagement even with initial uncertainty in the target location. However, many tagging devices depend on omni-directional radio frequency (RF) signals that can be intercepted or jammed. Therefore, other tagging devices relying on laser-based free-space-optical (FSO) communication have also been proposed, including ferroelectric liquid crystal modulator, micro-electromechanical (MEM) based corner cube, multiple quantum well retro-reflector, and thin-film diffractive modulating retro-reflector.
The ferroelectric liquid crystal modulator is a mature technology that provides wide field of view (FOV), wide operation wavelength differential Δλ, and low operation power. However, the ferroelectric liquid crystal modulator has the limited flexibility of liquid crystal display, a narrow operation temperature range between −10° C. to 60° C., and a low switching speed of about several kilobytes per second. The micro-electromechanical corner cube has a good contrast and requires very low operation power, but has limited switching speed and surface flatness, a small clear aperture and the very high fabrication cost. The multiple quantum-well retro-reflector has the advantages of high data rate of about several megabytes per second, a wavelength range safe to human eyes, and low operation power. However, it also has the disadvantages of narrow waveband, low modulation depth of about 1 dB, a small clear aperture (with a diameter smaller than 1 cm), and a high fabrication cost. The thin-film diffractive modulating retro-reflector provides the ultra-thin (about 1 mm) dimension, low cost, and modulation capability, but has the limitations of low reflection efficiency, a small field of view and a narrow optical bandwidth.
It is thus a substantial need to develop a non-RF dependent and non-visually-alerting optical tag which has a wide range of operation temperature, low operation power and low fabrication cost.